Beam systems, such as electron beam systems, ion beam systems, laser beam systems, cluster beam systems, and neutral particle beam systems, are used to create features on a surface by etching or depositing material. Beam-induced deposition processes typically use a precursor gas that reacts in the presence of the beam to deposit material on the surface in areas where the beam impacts. For example, a gaseous organometallic compound, such as tungsten hexcarbonyl, is provided near the sample and is adsorbed onto the surface. The organometallic compound decomposes in the presence of a beam, such as an ion beam or an electron beam, to form a metal that remains on the surface and a volatile organic compound that is removed by a vacuum pump. Because the precursor is stable, that is, it does not spontaneously decompose on the surface in the absence of the beam, a fine structure can be deposited, with the feature size of the structure determined by the beam size and the beam-sample interaction volume.
Two disadvantages of charged particle beam-induced deposition are low deposition rates and carbon contamination of the deposit. While charged particle beams can typically be made much smaller than a laser beam, the size of the beam spot on the work piece, is typically inversely proportional to the current in the beam. A small, high resolution beam, therefore, has a low current, which produces a low deposition rate. The rate of electron beam-induced deposition is typically between about 5×10−4 um3·nC−1 to about 5×10−3 um3·nC−1.
Precursor gases for charged particle beam deposition are typically carbon containing metallo-organic compounds. Carbon from the precursors, or from other sources such as lubricants in the vacuum chamber, typically contaminates the deposited metallic material, greatly increasing the resistivity of the deposit. The slow deposition rate and the high resistivity are undesirable for nanoprototyping and nanoresearch applications.
Various techniques have been tried to improve the purity of beam-induced deposits, but have met with limited success. Such techniques include heating the work piece during deposition, annealing the deposit after deposition, mixing the precursor gas with reactive gases during or after deposition, and using carbon-free precursors. An over view of techniques used to deposit a pure material is provided in A. Botman, et al., “Creating pure nanostructures from electron-beam-induced deposition using purification techniques: a technology perspective,” Nanotechnology 20 372001 (2009).
Pure deposits have been achieved in electron beam-induced deposition in ultra high vacuum systems, but such systems significantly increase cost and the deposit rate is still low. Contamination reduction has also been achieved in electron or ion beam-induced deposition by mixing a deposition precursor with an oxidizer such as O2 or H2O, as described, for example, in Folch et al, Appl Phys. Lett. 66, 2080-2082 (1995) and Molhave et al, Nano Lett. 3, 1499-1503 (2003)). Use of an oxidizer is often undesirable because it results in oxide formation or incomplete reduction of the precursor gas.
Attempts to use hydrogen gas to improve the purity and deposition rate of iron deposited from Fe(CO)5 have been unsuccessful, perhaps because of the low sticking coefficient and/or short residence times on common surfaces; H. Wanzenboeck et al, presented at International Conference on Electron, Ion, and Photon Beam Technology and Nanofabrication (EIPBN) 2008. The current state of the art of electron beam-induced deposition is described, for example, in Randolph et al, “Focused Nanoscale Electron-Beam-Induced Deposition and Etching,” Critical Reviews in Solid State and Materials Sciences, 31:55-89 (2006).